In today's vehicles, the brake lights are, in general, driven directly in dependence upon an actuation of a brake pedal.
DE-OS 20 40 337 discloses that a reduction of vehicle speed comes about not only by the actuation of a foot brake but also via other circumstances such as via a braking by the engine when there is a downshift operation in the transmission or exclusively by releasing the accelerator pedal. According to the contents of this publication, the vehicle deceleration is detected by a special acceleration sensor. The brake lights are then driven in dependence upon a deceleration when the latter occurs. A separate sensor of this kind is relatively complex and is, under circumstances, expensive and subject to malfunction.
The German utility model registration GM 91 08 827 shows a signal output device for controlling the brake light in a motor vehicle wherein the following is evaluated to determine a sudden reduction of vehicle acceleration: the position of the accelerator pedal actuated by the driver, the engine underpressure and/or the fuel throughput supplied to the engine. Here, it is disadvantageous that all of these quantities do not provide a reliable index for the vehicle deceleration.
An engine control system is known from U.S. Pat. No. 5,657,230 wherein the actual engine drag torque is determined in the engine control apparatus.